SAETTA: high resolution 3D mapping of the total lightning activity in the Mediterranean basin over Corsica, with a focus on a MCS event.

Deployed in the mountainous island of Corsica for thunderstorm monitoring purpose in the Mediterranean Basin, SAETTA is a network of 12 LMA stations (Lightning Mapping Array, designed by New Mexico Tech, USA) that allows the 3-D mapping of VHF radiations emitted by cloud discharges in the 60-66 MHz band. It works at high temporal (80 µs) and spatial (tens of meters at best) resolutions within a range of about 350 km. Originally deployed in May 2014, SAETTA was commissioned during the summer and fall seasons and is now permanently operational since April 2016 until at least the end of 2020. We first evaluate the performances of SAETTA through the radial, azimuthal, and altitude errors of VHF sources localization with the theoretical model of Thomas et al. (2004). We also compute on a 240 km x 240 km domain the minimum altitude at which a VHF source can be detected by at least 6 stations by taking into account the mask effect of the relief. We then report the 3-year observations on the same domain in terms of number of lightning days per square kilometer and in terms of lightning days integrated on the whole domain. The lightning activity is first maximum in June because of daytime convection driven by solar energy input, but concentrates on a specific hot spot in July just above the crossroad of the three main valleys. This hot spot is probably due to the low-level convergence of moist air fluxes from sea breezes channeled by the three valleys. Lighting activity increases again in September due to numerous small thunderstorms above the sea and to some high precipitating events. Finally we report lightning observations of unusual high altitude discharges associated with the mesoscale convective system of June 8, 2015. Most of them are small discharges on top of an intense convective core during convective surges. They are considered in the flash classification of Thomas et al. (2003) as small-isolated and short-isolated flashes. The other high altitude discharges, much less numerous, are long range flashes that develop through the stratiform region and suddenly undergo upward propagations towards an uppermost thin layer of charge. This observation supports the recent conceptual model of Dye and Bansemer (2019) that explains such upper level layer of charge in the stratiform region by the development of a non-riming ice collisional charging in a mesoscale updraft. [ABSTRACT FROM AUTHOR]